1. Field of Invention
The invention relates to a method of manufacturing a ceramic/metal composite structure, and more particularly to a method of manufacturing a composite structure composed of an aluminum oxide layer and a copper layer.
2. Related Art
Heat is generated when an electronic component is operating with electrons flowing therethrough. The generation of heat increases the resistance and blocks the flows of the electrons so that the function of the electronic component is significantly influenced. When the technology of manufacturing the electronic component is greatly enhanced, a line width in the electronic component is getting smaller and smaller, and the line density in the electronic component is getting higher and higher. Thus, the heat generated by the electronic component is increased rapidly. Taking a central processing unit (CPU) of a computer as an example, the Pentium CPU only has to be equipped with the package with the heat dissipating capability of 16 W at its early stage. However, the heat generated in the CPU in the year of 2004 has reached 84 W, and the heat generated in the CPU in the year of 2006 has reached 98 W. If the heat cannot be removed rapidly, the temperature of the CPU of the computer is rapidly increased so that the CPU of the computer can no longer operate. Thus, the heat dissipating ability of the substrate contacting with the CPU of the computer is a key factor for dominating whether the computer can operate normally or not.
A typical power component, such as a solid relay, is similar to the CPU of the computer and generates a lot of heat. Thus, the power component also dissipates the heat rapidly through the substrate contacting therewith so that it can operate normally.
Taking a light-emitting diode (LED) as another example, various color LEDs have been gradually developed, wherein the successful development of the white-light LED has attracted considerable attention. This is because the white-light LED can serve as a light source for an illumination lamp. The power consumption of a road lamp with LED light source is lower than that of a mercury lamp by 75% and is lower than that of a high pressure sodium lamp by 49%. So, the white-light LED advantageously has the low power consumption and can significantly save the energy. However, the white-light LED with the output power higher than 3 W has to be adopted in the application of the lamp used in the daily life and the applications such as the head light used in a vehicle. This white-light LED with the high output power also generates a lot of heat. However, the major barrier on the application of LED as the light source is that the LED cannot withstand the high temperature. Generally speaking, the temperature of the LED cannot exceed 90° C. If the temperature of the LED is higher than 90° C., the luminance thereof rapidly deteriorates. So, the rapid heat dissipating ability of the heat dissipation substrate in contact with the LED has become a greatest challenge for determining whether the LED can become the illumination light source or not. This also specifies that the development of the heat dissipating substrate has played an important role on the applications of the LED as the light source.
In order to satisfy the miniaturized requirement of the current 3C electronic product, the substrate contacting with the CPU of the computer, or the power component or the LED has to satisfy the following fundamental requirements.
First, the material must have a high heat conductivity to dissipate the heat rapidly.
Second, the material must have the high resistivity in order to prevent the high power electronic component from being short-circuited.
Third, the substrate preferably has to be as thin as possible after the above-mentioned conditions are satisfied.
Fourth, the substrate has to be used with the good reliability for a long time. This is because the high power electronic component, after being packaged, will encounter several tens of thousand times of on-off cycles, and the temperature of the substrate contacting with the high power electronic component is instantaneously increased and decreased therewith for several tens of thousand times. So, the reliability after the electronic component has been used for a long time is an extremely important requirement. This is absolutely associated with the bonding strength between the ceramic material and the metal material.
At present, the heat dissipating mechanisms of the electronic components, which are frequently used, include heat dissipating fins and a heat pipe accompanied with fans in order to dissipate the heat generated by the high power electronic components. However, such heat dissipating structure has a larger thickness, and the applications in designing a miniaturized 3C electronic product are thus hindered.
In order to satisfy the requirements on heat dissipating and to keep the size small and the price low, all materials are searched and evaluated. Taking the copper as an example, the copper has a high coefficient of thermal conductivity, which can reach 380 W/mK. There are many choices, which can satisfy the second requirement of insulation, because most polymeric organic materials or ceramic materials can satisfy this requirement. In order to satisfy the requirement of heat dissipating, the ceramic material is the better choice under the consideration of the long-term reliability. Among the ceramic materials, aluminum oxide and aluminum nitride can provide the high thermal conductivity and the high insulating resistance. The aluminum oxide has a coefficient of thermal conductivity, which can reach 20 to 38 W/mK, while the aluminum nitride has a coefficient of thermal conductivity, which can reach 40 to 200 W/mK. The coefficient of thermal conductivity of the ceramic material has a wider range because the coefficient of thermal conductivity of the ceramic material is significantly influenced by the purity of the ceramic material and the type of the sintering additive used. Furthermore, the resistivity of each of the aluminum oxide and the aluminum nitride can be equal to or greater than 1010 Ωm. Thus, the two ceramic materials have the excellent insulating properties. Furthermore, aluminum oxide and aluminum nitride have the low dielectric constant and the high dielectric strength, and are thus frequently used as the material for a substrate.
However, the aluminum oxide is a solid, which has a high melting point (higher than 2000° C.) and has the coexisted covalent bonds and ionic bonds. The copper atoms are combined with metallic bonds, and the copper has the melting point, which is only 1083° C. So, it is a great challenge to bond the aluminum oxide and the copper together. According to the report of Beraud, C., Courbiere, M., Esnouf, C., Juve, D., Treheux, D., J. Mater. Sci., 24, 4545, 1989, there are two conventional methods for bonding the aluminum oxide with the copper. The first method is the solid state bonding method, and the second method is the liquid phase bonding method, as disclosed in U.S. Pat. No. 3,993,411. The treating temperatures of these two methods are higher than 1000° C.
According to the research made by Seager et. al., (Seager, C. W., Kokini, K., Trumble, K., Krane, M. J. M., Scripta Materialia, 46, 395, 2002), the thick copper oxide is disadvantageous to the bonding between aluminum oxide and copper. After a long-term investigation, it is found that the substrate, formed by bonding an aluminum oxide sheet and a copper sheet together, cannot have the application value until the interface strength between the aluminum oxide sheet and the copper sheet reaches a very high level. This is because the aluminum oxide and the copper have different bonds, and the coefficient of thermal expansion of the copper (17×10−6K−1) is two times of the coefficient of thermal expansion of the aluminum oxide (8×10−6K−1). A formula derived by Selsing (Selsing, J., J. Am. Ceram. Soc., 44, 419, 1961) is listed in the following:
  σ  =                    Δ        ⁢                                  ⁢                  α          ⁢                                          ·          Δ                ⁢                                  ⁢        T                                          1            +                          v                              Al                ⁢                                                                  ⁢                2                ⁢                O                ⁢                                                                  ⁢                3                                                          2            ⁢                          E                              Al                ⁢                                                                  ⁢                2                ⁢                                                                  ⁢                O                ⁢                                                                  ⁢                3                                                    +                              1            -                          2              ⁢                              v                Cu                                                          2            ⁢                          E              Cu                                            .  In the above-mentioned formula, Δα denotes a difference between coefficients of thermal expansion of the aluminum oxide and the copper, ΔT denotes a difference between the room temperature and the manufacturing temperature, υ denotes the Poisson's ratio and E denotes the elastic constant. The temperature, at which the aluminum oxide sheet and the copper sheet are joined together is over 1000° C., so the estimated thermal stress induced by the thermal expansion mismatch between aluminum oxide and copper after the high temperature joining process can reach several hundreds of MPa. This thermal stress is very large and significantly influences the bonding strength between the aluminum oxide sheet and the copper sheet. In addition, after the aluminum oxide sheet and the copper sheet are joined together, the composite substrate is packaged together with electronic components. Since the electronic component may be turned on and off for several tens of thousand times, de-bonding may be formed at the interface between the aluminum oxide sheet and the copper sheet if the bonding strength between the aluminum oxide sheet and the copper sheet is not high enough. Thus, the heat spreading ability is greatly reduced, which significantly influences the reliability after the high power electronic component is used for a long time.
Thus, it is an important subject of the present invention to provide a ceramic/metal composite structure having a high bonding strength.